Paccar is the parent company for Peterbuilt and Kenworth.
The only difference I have found between Cummins and Paccar is the name stamped on the valve cover. Although newer truck components are being designed and assembled to work as a system. For example, a 32,000 truck with a 250hp engine will be matched to a transmission that is matched to the correct rated drive axle for weight and gear ratio, that is matched to the correct tire size, that is matched to the trucks intended use. Think of it as a circle that one part runs into the next.
The only reason I would think an independent Cummins shop may turn down work on a Paccar engine may be getting paid for warrantee work or if they are using NON OEM software.
Most aftermarket diagnostic computers are limited if not useless for several years at least 5 years if not longer after OEMs release new engines, etc., as the OEMs have allot of money wrapped up into their technology and manufacturing that they need to make back. Add to the fact manufacturers of diagnostic software and computers know that they will have the patent for a long time and will exploit that money train as long as possible.
I would check with a dealer service manager that services Cummins rather than an independent shop on this one.
Good Luck.
Bob Wolf